Remove Calendar from Conpherence

Summary: Ref T8834, cleanly removes "Calendar" widget from Conpherence. RIP. :(

Test Plan: Bounce around Conpherence, no Calendar. grep for "calendar" in apps folder, css, js

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T8834

Differential Revision: https://secure.phabricator.com/D13934